1. Field of the Invention
The present invention relates to an image display device, and more particularly, to a technology that is effective in transmitting/receiving display data via a non-contact transmission path.
2. Description of the Related Art
In an active matrix display device typified by a liquid crystal display device, a thin film transistor (hereinafter, referred to as TFT) constituting an active element is formed in each pixel. Display information is stored in each pixel, whereby an image is displayed.
On the other hand, a TFT having the following structure is called a poly-silicon TFT. That is, an amorphous silicon film is poly-crystallized by laser annealing, and a poly-silicon film having a mobility enhanced to approximately 100 cm2/V·s is used as a semiconductor layer.
A circuit including such a poly-silicon TFT operates in response to a signal having several MHz to several tens MHz at maximum. Therefore, in addition to pixels, a data driver circuit for generating an image signal and a scanning circuit for performing scanning can also be formed in the same process as TFTs constituting pixels on a board of the liquid crystal display device or the like.
Further, as a communication method in which a signal (such as display data or display control signal) is transmitted/received via a non-contact transmission path, there is known a communication method employing electrostatic induction (electrostatic capacitance coupling), electromagnetic induction, or an electromagnetic wave.
Among communication methods in which a signal is transmitted/received via the non-contact transmission path, the communication method employing an electromagnetic wave or electromagnetic induction is more suitable for a long-distance communication. However, in that method, a display signal needs to be modulated and demodulated with the use of a carrier wave having a frequency by far higher than a transmission rate of the display signal. This leads to increase in power consumed by high-speed circuit driving or increase in area for a circuit element formation.
On the other hand, in the communication method employing electrostatic capacitance coupling as illustrated in FIG. 5 of Patent Document 1, a communication is limited to a shorter distance transmission. However, the method can be realized only with transmission electrodes because modulation and demodulation are not necessary. In addition, an area occupied by the electrodes can be kept small because circuits for modulation and demodulation are not necessary.
However, in order to increase a transmission rate for higher resolution, a plurality of non-contact transmission paths are required. Specifically, electrodes for electrostatic capacitance coupling need to be arranged in parallel on a board.
As methods for realizing the plurality of non-contact transmission paths, there are known a balanced transmission illustrated in FIG. 7A and a non-balanced transmission illustrated in FIG. 7B. In FIGS. 7A and 7B, reference symbols STX and RTX denote a transmitting circuit and a receiving circuit, respectively.
In the balanced transmission illustrated in FIG. 7A, two capacitors (C1) are provided for one channel (ch) In the non-balanced transmission illustrated in FIG. 7B, one capacitor (C1) is provided for one channel (ch), and a capacitor (C2) having sufficient capacitance for all the channels is provided in order to alternately short-circuit between reference voltages (SCOM and RCOM) of the transmitting circuit (STX) and the receiving circuit (RTX). In short, more than one electrode needs to be provided for one channel.
When the plurality of non-contact transmission paths are employed in order to increase the transmission rate for higher resolution, the electrodes occupy a large area on a panel, and a non-display region called a frame is enlarged. A larger non-display region as described above is not desirable because flexibility of an application product is decreased.